How many gigawatts of sand storage have been provisioned (in terms of electricity output, not thermal output)? How many years of operational experience do we have? In fact I'm not aware of any project using thermal sand for electric storage, only for district heating. Finland uses about 87TWh electricity annually, working out to about 230 GWh daily. I'd be a lot more convinced of it's viability for seasonal storage if they're able to provision at least a day of storage.

I don't doubt your critical reasoning ability, but I do doubt the wisdom of treating the challenge of provisioning and operating massive seasonal storage on the order of terawatts as a solved problem because of a few district heating projects.